1. Field of the Invention
The invention relates to a white-light emitting device, more particularly to a white-light emitting device applicable to a backlight module.
2. Description of the Related Art
There are usually two types of backlight modules suitable for use in a liquid crystal display, i.e., a direct type backlight module and a side-edge type backlight module. Conventionally, a cold cathode fluorescent lamp is used as a light source for either type of the backlight modules. However, the cold cathode fluorescent lamp has disadvantages, such as use of poisonous mercury, fragility, inferior CRI (color rendering index), etc.
A backlight module that uses a light emitting diode (LED) as the light source was developed to overcome the aforesaid disadvantages. As shown in FIGS. 1 and 2, a direct type backlight module 1 is mounted directly on a rear side 22 of a liquid crystal module 2. The liquid crystal module 2 has the rear side 22 and a front side 21 opposite to the rear side 22, and includes a color filter unit 23, a liquid crystal unit 24, and a glass substrate unit 25 along a direction from the front side 21 to the rear side 22. The backlight module 1 includes a housing 11, which defines a receiving space 12 and which has a bottom wall 111, a printed circuit board 14 disposed on the bottom wall 111 of the housing 11, and a plurality of light emitting units 13 disposed on and connected electrically to the printed circuit board 14.
Each of the light emitting units 13 includes a first LED 131 emitting green light, a second LED 132 emitting blue light, a third LED 133 emitting red light, and a fourth LED 134 emitting green light. The lights emitted from the first, second, third, and fourth LEDs 131, 132, 133, 134 are combined with each other in the receiving space 12 so as to produce white light that is emitted to the rear side 22 of the liquid crystal module 2.
Since four LEDs are required for each of the light emitting units 13 in the aforesaid prior art, the housing 11 requires a relatively large receiving space 12 for a given number of the light emitting units 13. Furthermore, a relative long distance from the bottom wall 111 to a top edge 112 of the housing 11 is required for combining the lights emitted from the LEDs 131, 132, 133, 134 to obtain white light. Therefore, the housing 11 of the prior art cannot be reduced in thickness advantageously.